The present invention relates to the field of radio communications and, more particularly, to a method and apparatus for locating wireless terminals.
There are a number of different types of communications systems that require information regarding the location of an object or element in order to operate efficiently.
For example, some wireless communications systems require information regarding the location of a receiving unit in order to efficiently route signals from a transmitting unit to the receiving unit, and vice versa.
In cellular communications systems, for example, the cellular network (i.e. the network of base stations) requires information regarding the identity of the cell in which a wireless terminal is located in order to efficiently route signals to and from the wireless terminal. Once the cell is identified, the cellular network can send a signal to the wireless terminal through a base station that provides coverage to the identified cell. This enables the cellular network to deliver the signal to the wireless terminal without having to transmit the signal through every base station. As a result, the cellular network is able to avoid consuming bandwidth in those cells in which the wireless terminal is not resident, and thus increase the overall efficiency of the cellular system.
Although present-day cellular networks identify the cell in which the wireless terminal is located, they do not identify the exact geolocation of the wireless terminal in that cell. The term geolocation as used herein refers to the point in two or three-dimensional space defined by a set of coordinates (e.g. longitude and latitude) and/or defined by a vector (i.e. distance and direction) from a known point in space. This lack of geolocation information greatly decreases the efficiency of the cellular system when the wireless terminal moves from cell to cell. In addition, the lack of geolocation information reduces the number of services that a given cellular system can provide (e.g. roadside assistance, fleet management, etc.).
To illustrate, cellular systems in North America presently use a so-called system-wide paging approach to identify the cell in which a wireless terminal is located. Pursuant to that approach, the wireless terminal periodically transmits identification information, referred to as registration, to the cellular network. Depending on the location of the wireless terminal, the registration signal can be received by any number of base stations, each base station covering a specific cell. The cellular network identifies the cell in which the wireless terminal is located by comparing the strength of the registration signal received at each base station. The base station that receives the strongest registration signal is identified as the cell in which the wireless terminal is presently located. Once the xe2x80x9cpresentxe2x80x9d cell is identified, the cellular system can then communicate with the wireless terminal through the base station that covers that cell.
If the wireless terminal moves out of the present cell, the cellular system must send a page to the wireless terminal through a plurality of cells and wait for the wireless terminal to send another registration. Once the new registration is received, the cellular system can then identify the new cell in which the wireless terminal is located by comparing the signal strengths, as described above. As a result, the cellular network must consume additional bandwidth to periodically identify the cell in which the wireless terminal is located.
In addition, if the user of the wireless terminal makes a call for help (e.g. because the user is in distress), the wireless network can not identify the geolocation from which the wireless user made the call. As a result, unless the user can accurately identify his or her location, the wireless network can provide only limited help in the dispatch of aid to the user. Thus, present-day cellular systems in North America, for example, are not very useful to other service providers, such as roadside assistance and medical emergency care.
One solution to this problem is to equip the wireless terminal with the ability to identify its own geolocation. For example, the wireless terminal can be equipped with a Global Positioning System (GPS) receiver that receives GPS signals and uses those signals to determine the geolocation of the wireless terminal. Once determined, the geolocation information can be periodically sent to the cellular network. This would enable the cellular network to periodically identify the geolocation of the wireless terminal without having to consume additional bandwidth sending pages. In addition, this solution would enable the cellular network to track the movement of the wireless terminal from cell to cell, and thus predict and/or anticipate the time to hand-off communications from one cell to the next. As a result, this solution would enable the cellular network to increase the overall efficiency of communications, and to provide additional information to service providers such as roadside assistance providers (e.g. to located a user in distress).
The just described solution, however, is disadvantageous because the additional hardware needed to equip the wireless terminal with the ability to identify its own geolocation would increase the price, size and weight of the wireless terminal to unappealing proportions. Indeed, there is an ongoing effort by those skilled in the art to reduce the price, size and weight of present-day wireless terminals (e.g. cell phones).
Techniques are known, outside the cellular communications arena, for identifying the geolocation of a transmitting unit. For example, in some satellite communications systems, the geolocation of a transmitting unit is identified by determining the times in which the line-of-sight components of a signal transmitted from the transmitting unit reached a respective receiver locations. The line-of-sight component of the so-called incoming signal at each receiver location is that component of the signal that propagated directly from the wireless terminal to the location at which the signal was received (i.e. the receiver location) without scattering or reflecting off structures in the environment. To determine the time-of-arrival of the line-of-sight component of the incoming signal, such satellite geolocation systems assume that the first-arriving component of the incoming signal is the line-of-sight component. Then, based on the time-of-arrival of the first-arriving component of the incoming signal at, for example, three receiver locations, the geolocation of the wireless terminal is calculated.
Such conventional geolocation systems, however, are hindered by their failure to consider the problems associated with scattering in the RF environment. Scattering refers to the phenomenon wherein signals traveling in an RF environment reflect off structures in the environment, and thus scatter in various different directions or paths in the RF environment. Specifically, scattering may cause a signal to travel more than one path between two points (e.g. a wireless terminal and a receiver location) in an RF environment. This so-called multipath phenomenon may cause an incoming signal at a receiver location to be composed of a plurality of so-called multipath components (i.e. repeated versions of the transmitted signal). Thus, depending on the propensity of the RF environment to scatter a signal, referred to herein as the scattering hostility, the incoming signal at the receiver location may be composed of a number of such multipath components.
Scattering and/or multipathing may cause the first-arriving component of the incoming signal to arrive at the receiver location very close in time to the time-of-arrival of the next-arriving multipath component of the incoming signal. A conventional geolocation system may not be able to distinguish between the two components, causing it to mistakenly determine that the time of arrival of the first-arriving component is at some intermediate time between the time of arrival of the first-arriving component and the time-of-arrival of the next-arriving multipath component, i.e., a later time than the first-arriving component actually arrived. This so-called time-shift of the identified time-of-arrival may cause the geolocation system to determine an incorrect geolocation of the wireless terminal.
In addition, depending on the scattering hostility of the RF environment, the line-of-sight path between a wireless terminal (i.e. the transmitting unit) and a base station (i.e. the receiver location) may be blocked (e.g. by a building). When this happens, the first-arriving component of the incoming signal (at the receiver location) will not be the actual line-of-sight component, but rather some later-arriving component. Again, this would cause the conventional geolocation systems to incorrectly assess the time-of-arrival of the line-of-sight component as being received at a later point in time, i.e. time-shifted, thereby causing the geolocation system to inaccurately calculate the geolocation of the wireless terminal.
As a result, it can be understood that there are at least two different mechanisms wherein scattering and/or multipathing may cause a so called time-shift of the determined time-of-arrival of the line-of-sight component of the incoming signal, and thus be problematic to the accuracy of identifying the geolocation of a wireless terminal by conventional geolocation systems.
The above-described problems are ameliorated in accordance with the principles of the invention. In particular, the geolocation of a wireless terminal is determined by a performing a time/frequency analysis of a signal transmitted by the wireless terminal during, for example, regular communications. This approach allows the terminal to be identified with more accuracy than is achieved by attempting to identify the time-of-arrival of the first-arriving multipath component of the incoming signal; without having to add substantial hardware to the wireless terminal, and without having to consume additional bandwidth on the wireless network with which the wireless terminal communicates.
The term time/frequency analysis as used herein refers to an analysis of the frequency components (i.e. the frequency make-up) of a signal at given instants in time. For example, one form of time/frequency analysis according to the present invention is to compare the frequency make-up of the received signal to the frequency make-up of the transmitted signal. Those points in time in which the frequency make-up of the received signal matches the frequency make-up of the transmitted signal are the instants in time at which a particular component of the transmitted signal is received.
In accordance with a feature of the invention, the time/frequency analysis is used to identify the time-of-arrival of the line-of-sight component of the incoming signal. By performing such a time/frequency analysis on the line-of-sight components of the incoming signals received at, for example, three locations, the geolocation of the wireless terminal can be identified with better accuracy than in the prior art because the identified time-of-arrival at each location is more accurate than that identified in the prior art.
In preferred embodiments, the accuracy of the identified time-of-arrival of the line-of-sight component of the incoming signal at each receiver location is improved by an amount based on the value of at least one parameter of an RF model that characterizes the scattering hostility of the RF environment in which the respective incoming signal traveled. By adjusting the time-of-arrival in this way, the above-discussed time-shift due to scattering is compensated for, and thus more accurately reflects the time at which the line-of-sight component of the incoming signal would have arrived if the RF environment were scatter-free. This increased accuracy of the identified time-of-arrival of the line-of-sight component of the incoming signal will cause an increase in the accuracy of the determination of the geolocation of the wireless terminal.
In particular embodiments, the time/frequency analysis may advantageously be carried-out using the signal processing disclosed in co-pending application Ser. No. 08/984,728, entitled xe2x80x9cMethod For Improved Line Of Sight Signal Detection Using Time/Frequency Analysis,xe2x80x9d filed on even date herewith.
In addition, in particular embodiments, an identified time-of-arrival of the line-ofsight component of the incoming signal can be adjusted using the methods disclosed in co-pending application, Ser. No. 08/984,780, entitled xe2x80x9cMethod For Improved Line-Of-Sight Signal Detection Using RF Model Parameters,xe2x80x9d filed of even date herewith.
Also, in particular embodiments, the parameters of the RF model used to adjust the identified times-of arrival can be advantageously carried-out using the methods disclosed in co-pending application Ser. No. 08/984,779, entitled xe2x80x9cMethod For Frequency Environment Modeling And Characterization,xe2x80x9d filed of even date herewith.